Ice Balloons

1. Stretch the neck of a balloon over a faucet and carefully fill with tap water. Work slowly to avoid pumping a lot of air into the balloon. When the balloon is at least 5 inches (25 cm) in diameter, remove it from the faucet, squeeze out the air at the top, and tie it closed.
2. Make as many water-filled balloons as you will need. Then put them in the freezer and allow them to freeze for at least two days.
3. When everything’s ready, fill a plastic tub with enough water to float an ice balloon. Don’t fill it all the way to the brim; leave room for the water to rise when you add the balloon.

Cut off the neck and peel the balloon off the ice.

Put the ice balloon on the cafeteria tray and start by just taking a close look at it. What do you notice? A few things to look for are clear ice near the surface of the balloon, bubbles inside (some elongated, some making chains), the opaque center, and frost forming and then disappearing from the ice balloon’s surface (click to enlarge the photo below).

Sprinkle a small amount of salt on top of the balloon—about half a teaspoon (2-3 ml). Then wait and watch. Notice how liquid water forms around the salt, creating meandering patterns as it flows down the balloon. Put a few drops of food coloring onto the salt to make the flow patterns more obvious (see photos below).

Finally, rinse off the salt and food coloring and put the ice balloon into the water basin. Notice how it floats. How much of the balloon is above the water level, and how much is below?

There’s plenty to learn from a frozen water balloon, starting with the patterns of bubbles—or lack thereof—in the ice.

The water in an ice balloon freezes from the outside in. As the water freezes, it creates pure crystals of water, which are clear. Meanwhile, impurities such as air or minerals are left behind in the liquid, where they concentrate until they come out of solution as bubbles. One bubble can seed a neighboring bubble, creating a radial chain of bubbles. Since bubbles scatter light of all wavelengths, they give the ice balloon a white, opaque center (see photo below).  

When the balloon comes out of the freezer, it’s often at a temperature of 0°F (-18°C), much colder than the freezing point of 32°F (0°C). At these cold temperatures, water vapor in the air can freeze onto the balloon, creating a layer of frost. When the surface of the balloon warms to the freezing point, a visible film of water appears on the surface and the frost disappears.

Salt on the balloon will cause the ice to melt, even at temperatures below freezing. In any ice/water combination, there is an ongoing back-and-forth in which some liquid water molecules are freezing while some solid water (ice) is melting. Ions of sodium and chlorine from the salt get in the way of ice-crystal formation, turning the back-and-forth into more of a one-way street in which more ice melts then freezes.

As the salty liquid water flows down the balloon, it begins to form meandering streams, just as rivers do (see photo below). As in rivers, the meanders shift over time, responding to subtle changes in flow and channel shape.

Most substances shrink as they cool, but water is a notable exception, freezing into hexagonal crystalline structures that take up about 10 percent more space than liquid water. This increased volume translates into lower density, causing ice to float. A solid ice balloon placed in water displaces its weight in water—this is Archimedes’ principle—with 10 percent of the ice balloon above the surface and 90 percent below.

Try adding sugar to the ice balloon. How is the result different from adding salt?

If a water faucet it not easily available, you can also fill an ice balloon from a two-liter bottle. Just stretch the mouth of the balloon over the top of the bottle and squeeze in the water.